[0001] Several methods for synthesizing N,N-dialkyl-hydroxylamines are already known. Particularly,
the nitrones can be reduced either to disubstituted hydroxylamines by means of lithium
aluminum hydride or by means of potassium boro-hydride or by hydrogenation on platinumblack
(Coll. Czech. Chem. Comm.
2C, 202 (1955), JACS
79, 5739 (1957);
78, 6208 (1956); Gazz. Chim. Ital.
51, II, 306 (1921)). The pyrolysis of trialkylamine oxides, known as Cope reaction,
is useful for the synthesis of N,N-dialkyl-hydroxylamines as well.
[0002] Should the amine oxide have more than one alkyl group capable of forming an olefin,
a mixture of hydroxylamines is obtained (Org. Synthesis Coll. Vol. IV; 612 (1963)).
[0003] The N,N-dialkyl-hydroxylamines may also be prepared either by reaction of compounds
containing a N-O bond, by allowing said compounds to react with organometallic compounds
(J.Chem. Soc.
119, 251 (1921)) or by alkylation of hydroxylamines or N-alkylhydroxylamines with alkyl
halogenides (J. Org. Chem.
28, 1968 (1963); US-A-3491151; C.A.
72, 132130 (1970)). It is also known that secondary amines, when treated with hydrogen
peroxide or with acylperoxides, give rise to the formation of N,N-dialkyl-hydroxylamines
(Chem. Ber.
65, 1799 (1932); Arch. Pharm.
299, 166 (1966); JACS 72, 2280 (1950); J. Chem. Soc. 3144 (1963)). The reaction is of
a general type and can be used with primary amines as well. The modalities involved
up till now proved to be, however, extremely unsatisfactory, owing the low yield in
the desired product. Moreover, the oxidation of the carbon atoms in alpha position,
with respect to nitrogen, gave rise to the formation of a complex mixture of products.
The oxidation of secondary amines with hydrogen peroxide was carried out in the presence
of a reaction promoter as well, in particular in the presence of an ester of formic
acid (DE-A-1004191) or in the presence of a usual catalyst, containing Mo, W and the
like (BE-A-615,736). In each case the yields are low, whereas the decomposition of
hydrogen peroxide clearly prevails over the formation of hydroxylamine. Also the above
mentioned preparation processes for N,N-dialkylamines which are not based on oxidation
by means of hydrogen peroxide, are characterized by the need for expensive reactants,
by the involvement of not very stable compounds, by the formation of a large number
of by-products and difficulties in isolating the desired product.
[0004] It has now surprisingly been found that the preparation process for N,N-dialkylhydroxylamines
(in particular of N,N-diethyl-hydroxylamine) can be very much improved by carrying
out the oxidation of the corresponding amine by means of hydrogen peroxide in the
presence of a particular catalyst.
[0005] In its broadest sense the invention resides in a process for the synthesis of N,N-dialkyl-hydroxylamines
of general formula (I):

wherein R₁ and R₂, which may be the same or different, represent an alkyl, cycloalkyl,
alkyl-cycloalkyl or cycloalkylalkyl group having from 1 to 40 carbon atoms or are
part of a cycloaliphatic ring containing the hetero-atom N and having from 4 to 8
carbon atoms, by reaction with hydrogen peroxide of the corresponding (secondary)
dialkyl-amines of general formula (II):

which is characterized in that said reaction is carried out in the presence of a
catalyst based on titanium-silicalite.
[0006] The term "titanium-si[icalite" is described and defined in e.g. European patent application
88111138 and in EP-A-267362 and in the documents mentioned therein. The disclosure
of said applications, as far as it relates to titanium-silicalites, is incorporated
herein.
[0007] Extraordinary results may be obtained starting from secondary amines in which R₁
and R₂ each contain from 1 to 8 carbon atoms, or from heterocyclic compounds of pyrrolidinic
or piperidinic nature, particularly diethyl-amine, dipropyl-amine, dioctylamine; N-methyl-,
N-ethyl-amine and pyrrolidine.
[0008] Further examples of preferred groups R₁ and R₂ are: butyl, pentyl, hexyl, heptyl
(alkyl groups may be straight-chain or branched); cyclopentyl, cyclohexyl, cyclooctyl;
methylcyclopentyl, dimethylcyclopentyl, methylcyclohexyl, dimethylcyclohexyl; cyclopentylmethyl
and cyclohexylmethyl. When R₁ and R₂ combine to form a heterocyclic ring they may
represent e.g. butylene, pentylene, hexylene and 3-oxa-pentylene.
[0009] The practical interest in hydroxylamines is due to their use in different fields
as reducing agents, stabilizers or polymerization inhibitors.
[0010] In particular, on account of these specific properties, these compounds may satisfactorily
replace hydrazine derivatives as deoxygenating agents for water which is to be fed
into thermal plants or in general into steam generating plants. By the process according
to the invention a high yield with respect to the hydrogen peroxide (generally between
85 and 95%), a high selectivity with respect to the starting amine (higher than 90
and sometimes even higher than 95%) and a practically quantitative conversion of the
starting amine can be obtained.
[0011] The hydroxylation process for the secondary amines by means of H₂O₂ may be carried
out in different ways; for instance, it may be operated either in the absence or in
the presence of a solvent, said solvent being e.g. water or a suitable organic solvent
miscible with water, such as the aliphatic alcohols, or mixtures thereof. Particularly
good results have been obtained by using as solvent a tertiary alcohol which is practically
inert with respect to the oxidizing system; really extraordinary results have been
obtained with t-butyl or t-amyl alcohol. The temperature generally ranges from 25
to 150°C, preferably from 40 to 120°C. The hydroxylation of the dialkylamines may
generally be carried out at atmospheric pressure or (preferably) at superatmospheric
pressure, in order to keep both solvent and reactants in the liquid phase. The catalyst
is preferably used in a form finely dispersed in the reaction medium, in amounts ranging
from 0.1 to 50 parts by weight (preferably from 1 to 30 parts) per 100 parts of dialkylamine.
The weight ratio of dialkylamine to solvent generally ranges from 1 to 30 parts, preferably
from 1 to 20 parts by weight of amine per 100 parts of solvent. The reaction stoichiometry
requires an amount of hydrogen peroxide equivalent to the amount of amine; generally
the molar ratio of the two reactants (hydrogen peroxide: dialkylamine) ranges from
0.9 to 1.2, preferably from 0.9 to 1.1.
[0012] The process according to the invention may be carried out either in a semicontinuous
way (by feeding only hydrogen peroxide continuously to the reactor) or in a continuous
way (by continuously feeding both the reactants to the reactor). The reactor effluent
is a suspension that needs to be filtered in order to recover the catalyst (which
may be recycled to the reaction); if the filtration is carried out inside the reactor
the recovered effluent consists of a solution of the starting amine, the reaction
product, the reaction water and the solvent. The different components can be isolated
from this solution by known methods (distillation, crystallization, extraction and
the like). Non-converted reactants and solvent may be recycled to the hydroxylation
reaction, whereas the reaction product is recovered and optionally subjected to other
purification operations, according to the desired degree of purity.
[0013] The following examples illustrate the invention, without limiting, however the scope
thereof.
Example 1
[0014] A glass reactor equipped with a stirrer and heating jacket was pressurized with nitrogen,
after a vacuum had been applied by means of a mechanical pump; said reactor was charged
with 1.5 g of a finely subdivided powder, obtained by grinding a titanium silicalite
(prepared according to example 2 of EP-A-267362) with 7.21 g of diethylamine in 50
ml of t-butyl alcohol. The temperature was gradually increased by feeding a thermostatic
liquid of a temperature of 80°C to the reactor jacket. At this point hydrogen peroxide
(as a 30% by weight aqueous solution/was added. The addition was carried out over
35 minutes, adding a total of 5.97 g of dilute H₂O₂, corresponding to 0.056 moles
of pure H₂O₂. Thereafter the solution was cooled and directly analysed. The non-converted
diethylamine accounted for 3.49 g, whereas the formed N,N-diethylhydroxylamine amounted
to 4.32 g, which corresponds to a 51.5% conversion, with a 95.5% selectivity to N,N-diethylhydroxylamine;
the hydrogen peroxide conversion was practically complete, with a yield to N,N-diethylhydroxylamine
of 87.1%.
Example 2
[0015] Example 1 was repeated, increasing the hydrogen peroxide amount to 9.63 g, corresponding
to 0.090 moles, and by carrying out the addition over 54 minutes. The obtained results
were as follows:
- diethylamine conversion |
80.4% |
- amine selectivity to N,N-diethyl-hydroxylamine |
92.3% |
- N,N-diethyl-hydroxylamine yield (with respect to H₂O₂) |
80.9% |
- hydrogen peroxide conversion |
99.8% |
Example 3
[0016] Example 2 was repeated, increasing the t-butanol amount to 100 ml and keeping unchanged
the other reactants and the reaction conditions. The obtained results were as follows:
- diethylamine conversion |
84.5% |
- amine selectivity to N,N-diethyl-hydroxylamine |
88.7% |
- N,N-diethyl-hydroxylamine yield (with respect to H₂O₂) |
78.3% |
- hydrogen peroxide conversion |
99.4% |
Example 4
[0017] Example 1 was repeated, adding 25 ml of H₂O₂ and 25 ml of t-butanol (as the dispersing
medium for the catalyst) and changing the reaction temperature (to 60°C); the obtained
results were as follows:
- diethylamine conversion |
40.9% |
- amine selectivity to N,N-diethyl-hydroxlamine |
54.2% |
- N,N-diethyl-hydroxylamine yield (with respect to H₂O₂) |
41.1% |
- H₂O₂ conversion |
91.6% |
Example 5 (comparative example)
[0018] Example 1 was repeated, omitting the addition of the catalyst. The obtained (very
poor) results were as follows:
- diethylamine conversion |
23.4% |
- selectivity to N,N-diethyl-hydroxylamine |
17.3% |
- N,N-diethyl-hydroxylamine yield (with respect to H₂O₂) |
7.2% |
- hydrogen peroxide conversion |
66.9% |
Example 6
[0019] The apparatus used in Example 1 was charged with 7.4 g of pyrrolidine, 50 ml of t-butyl
alcohol and 1.5 g of the same (finely subdivided) titanium-silicalite. The suspension,
kept under stirring by means of a magnetic stirrer, was heated at 80°C. Thereafter
the addition of dilute hydrogen peroxide (at 30% by weight) by means of a metering
pump was commenced. The addition was completed within 150 minutes; the total amount
of added H₂O₂ was 0.054 moles. When the addition was over, the solution was cooled
and analysed. The obtained results were as follows:
- pyrrolidine conversion |
30.5% |
- pyrrolidine selectivity to N-hydroxy-pyrrolidine |
30.4% |
- N-hydroxy-pyrrolidine yield (with respect to H₂O₂) |
18.0% |
- hydrogen peroxide conversion |
99.7% |
Example 7 (comparative example)
[0020] Example 6 was repeated without using any catalyst; the obtained (very poor) results
were as follows:
- pyrrolidine conversion |
25.4% |
- pyrrolidine selectivity to N-hydroxy-pyrrolidine |
0.2% |
- N-hydroxy-pyrrolidine yield (with respect to H₂O₂) |
0.1% |
- hydrogen peroxide conversion |
93.6% |
1. Process for the synthesis of N,N-dialkyl-hydroxylamines of general formula (I)

wherein R₁ and R₂, which may be the same or different, represent an alkyl, cycloalkyl,
alkyl-cycloalkyl or cycloalkyl-alkyl group having from 1 to 40 carbon atoms or are
part of a cycloaliphatic ring containing the hetero-atom N and having from 4 to 8
carbon atoms, by reaction with hydrogen peroxide of the corresponding (secondary)
dialkyl-amines of general formula (II)

characterized in that said reaction is carried out in the presence of a catalyst
based on titanium-silicalite.
2. Process according to claim 1, wherein said secondary amine contains alkyl groups
having from 1 to 8 carbon atoms or is a heterocyclic compound of pyrrolidinic or piperidinic
nature.
3. Process according to claim 2, wherein said amine is selected from the group of
dimethylamine; diethylamine: N-methyl, N-ethyl-amine; dipropyl-amine; dioctylamine
and pyrrolidine.
4. Process according to anyone of the preceding claims, wherein said reaction is carried
out in a solvent selected from the group of water, aliphatic alcohols and mixtures
thereof, in a weight ratio amine: solvent of from 1:100 to 30:100, the reaction temperature
ranging from 25 to 150°C.
5. Process according to anyone of the preceding claims, wherein the amount of catalyst
ranges from 0.1 to 50 parts by weight per 100 parts of amine.
6. Process according to anyone of the preceding claims, wherein the H₂O₂:amine molar
ratio ranges from 0.9 to 1.2.
7. Use of N,N-diethyl-hydroxylamine as deoxygenating agent for water to be fed into
thermal plants or into steam generating plants.